OK, I haven't been yet, but we will be going in April, so I've done some research. The "must do" attractions also are the ones with long lines......the top 3 appear to be Pirate of the Caribbean, Jungle Cruise, the Pinball game (mighty ducks, I think) and the "Create your own Rollercoaster" attraction.
Rainy Days are crowded. January rainy days (hence, cold too) would probably be REALLY crowded. Recommendation is to get there right at opening. The later in the day, the more crowded it is.
Not sure about motion sickness, although I think I remember hearing the roller coaster one can be bad if you get too carried away with your design!